Chemically induced superplastic deformation

ABSTRACT

The invention produces superplastic deformation in a workpiece by altering the chemical composition of the workpiece material, while the workpiece is subjected to a biasing stress, in a manner that introduces a strain increment into the material that effects a change in a overall dimension of the workpiece without causing failure. In one approach, repeated cyclic alteration of chemical composition, so as to repeatedly alternately induce and reverse a phase transition that produces strain increment, allows accumulation of strain in an incremental fashion thereby achieving large overall superplastic deformations in the workpiece without applying large stresses.

This invention was made with government support under United States Armycontract #DAAH04-95-1-0629. The government has certain rights in thisinvention.

FIELD OF THE INVENTION

This invention relates to superplastic deformation. More particularly,this invention relates to a technique for inducing superplasticdeformation by chemical means.

BACKGROUND OF THE INVENTION

Superplastic deformation is defined as the deformation of a workpiece toa very large strain by application of a small stress without disruptingthe mechanical integrity of the workpiece. Although superplasticdeformation is universally characterizable by the formula ##EQU1## (inwhich ε is strain rate, A is a materials constant, σis stress, R is thegas constant, T is temperature and n is a stress exponent between oneand two), this behavior can be produced by any of several differentmechanisms. This phenomenon has been exploited in superplastic formingtechniques. For example, titanium-based materials are desirable fortheir specific strength and stiffness at ambient and elevatedtemperatures but have high resistance to deformation at temperaturesappropriate for traditional hot-working operations. However, titaniumalloys having a fine, stable grain structure deforms superplastically, aphenomenon known as "fine-grain superplasticity". Titanium-formingtechniques based on fine-grain superplasticity only operate successfullywithin a restricted window of process parameter values. For example,only small strain rates can be imposed, so the process output rate islimited. The deformation mechanism requires that grain size bemaintained within certain limits throughout the deformation process.

In another superplastic mechanism, called "transformationsuperplasticity" (described, e.g., in U.S. Pat. No. 5,413,649, theentire disclosure of which is incorporated herein by reference), theworkpiece is cycled through a phase transformation by changing thetemperature. The technique is advantageous compared to earlierapproaches in that it is not limited to a workpiece material with afine-grain microstructure and the grain growth limitation is relaxed.Also, the higher strain rates achievable result in more efficientprocess output. However, prolonged residence at high temperatures asrequired for some thermal cycling procedures can promote grain growth tosizes deleterious to the mechanical properties of the finished product.Implementing the required temperature cycling capability can be costlyand difficult. Also, repeated thermal cycling can promote fatigue of thetreatment apparatus.

DESCRIPTION OF THE INVENTION OBJECTS OF THE INVENTION

An object of the invention is, accordingly, to provide a technique forinducing superplasticity that is applicable to a wide range of workpiecematerials, including titanium-based materials.

Another object of the invention is to provide a technique for inducingsuperplasticity that is not limited to any specific workpiecemicrostructure or composition.

Another object of the invention is to provide a technique for formingcomposites.

Another object of the invention is to provide a method of inducingtransformation superplasticity without thermal cycling.

Another object of the invention is to provide a method of inducingsuperplasticity that allows fast deformation of the workpiece.

Still another object of the invention is to provide a method of inducingsuperplasticity that may be applied repeatedly to a workpiece withaccumulation of deformation from each repetition.

BRIEF SUMMARY OF THE INVENTION

The method of the invention produces superplastic deformation in aworkpiece by altering the chemical composition of the workpiecematerial, while the workpiece is subjected to a biasing stress, in amanner that introduces a strain increment into the material and therebyeffects a change in a overall dimension of the workpiece, withoutcausing failure. Depending on the material, the strain increment can begreater than 0.5% or 1%, even as much as 1.5% and greater. Knownapparatus for fine-grain superplastic forming can be modified in astraightforward manner to incorporate the method of the invention byadding a mechanism for introducing and/or withdrawing a chemicalcomponent to effect the desired chemical composition change. The presentinvention may also be used for compacting a workpiece initiallycomprising several distinct bodies (e.g., powder, wires, foils) to forma dense article or for foaming a workpiece by the expansion of internalcavities.

The alteration in composition may be monotonic, either resulting in apermanent change in the concentration of the component or reversed aftercompletion of the superplastic deformation process. Or the alterationmay be cyclic, comprising an initial increase or decrease in theconcentration of the chemical component, followed by a partial or totalreversal of the initial change while the workpiece remains subject tothe biasing stress.

In one approach, the composition changes within a single-phase stabilityfield, a concomitant change in lattice strain producing the strainincrement without phase transformation. In another approach, thealteration in composition induces a phase transition that gives rise tothe strain increment.

Such a change in composition in the material may affect all of theworkpiece material or only a part of it. The overall deformation isusually proportional to the fraction of the workpiece involved in thealteration. As used in this document, the term "segment" refers to theportion of the workpiece material undergoing a composition change and/ora phase transformation, whether it corresponds to the entire workpieceor not. The segment may, for example, form a continuous layersurrounding an unaltered core or be a collection of distinct isolatedregions, each surrounded by unaltered material. In the case of phasetransition, each forward or reverse transformation changes thetransformed segment with respect to some aspect--its specific volume or,in some instances, some geometric aspect such as lattice type, latticeorientation or shape--so that the transformation generates an internaltransformation stress in the material. In the case of chemicalcomposition cycling it is usually desirable that the segment transformedby the reverse transformation correspond to that transformed by theforward transformation, so that the original phase constitution of thematerial is completely restored. However, the invention does not requiresuch a correspondence; some of the material may remain in theforward-transformed state at the end of a cycle.

The transformations may occur along a macroscopic transformation frontbetween an original phase in the material and a new phase in thematerial, originating in the reaction where the chemical compositionchange is introduced and advancing into the material in an organizedfashion; or they may arise simultaneously at several discrete sites,having phase boundaries that move in random directions duringtransformation.

The scope of the invention is not limited with respect to type of phasetransition or workpiece material. The phase transition may involveprecipitation of a compound due to solute saturation or be, for example,allotropic, martensitic, peritectoid or eutectoid in nature. The methodof the invention is compatible with, but not limited to, metallic ionicand covalent materials including pure metals and alloys, such asintermetallics, ceramic, polymeric or geologic workpiece materials.

The biasing stress influences the orientation of the strain increment toproduce the desired superplastic deformation. The biasing stress mayoriginate in a source either internal to or external to the sample; or,both internal and external sources may contribute to the bias. Residualinternal stress in the workpiece may provide the biasing stress or, thetransformation stress of the phase transition may itself give rise tothe bias. In a preferred embodiment, the bias is provided by anexternally applied stress, the magnitude of which is chosen according tothe strength of the material. Depending on the deformation desired, theexternally applied biasing stress may be hydrostatic or nonhydrostatic,such as a uniaxial or multiaxial stress. Such stresses may includetensile, compressive, noncompressive, torsional or bending stresses asare conventionally used to effect, for example, drawing, punching,stamping, extruding, rolling, pulling, bending, and twisting.

In a preferred embodiment, chemical composition cycling is applied tothe workpiece repeatedly, each repetition introducing a strainincrement. Repetitive cycling in a manner that causes the alternateinduction or reversal of a phase transition to repeat is especiallybeneficial. The strain increment per cycle may be as much as 1.5%, orgreater. The accumulation of strain in this incremental fashion allowsachievement of large overall superplastic deformations in the workpiecewithout applying large stresses, which would risk disruption of themechanical integrity of the workpiece. The invention does not requirethat a chemical composition change applied or segment affected in anygiven cycle correspond exactly to that transformed in any other cycle ofa repetitive series.

Although the invention is applicable to a wide range of workpiecematerials, alterable by a commensurately broad range of compositionalchanges, superplastic deformation is most efficiently accomplished ifthe compositional change is imposed by varying the concentration of achemical component that has a high diffusivity in the workpiece materialbefore and after the ensuing phase transformation. As a practicalmatter, it is desirable that the component be easily transportable toand removable from the surface of the workpiece. It is thus preferablethat the chemical component be transported in the gas phase or producedby reaction at the workpiece of a species delivered in the gas phase.Such a component can then be removed by exposing the workpiece to vacuumor to another gas with zero or reduced pressure of the component, or byproviding a getter to absorb the gaseous species. It is furtherpreferable that small changes in the concentration of the chemicalcomponent produce a significant strain increment.

Using the method of the invention, it is possible to obtain thesuperplastic effects of phase transformations previously exploited bythermal cycling--such as allotropic phase transformation between alower-temperature phase and a higher-temperature phase--withoutdeliberate imposition of heating or cooling operations. This approachsimplifies the control equipment required to operate the treatmentapparatus and decreases its energy consumption. Related benefits arereduced risk of thermal fatigue of the treatment apparatus and reducedrisk of undesirable grain growth in the workpiece material.

Introduction of an alloying element that shifts the composition of theoriginal material sufficiently so that at least some of the originalmaterial converts to a different allotropic form is one way to induce aphase transition in accordance with the invention. For example,hydrogen, vanadium and niobium are known to be beta-phase stabilizersfor titanium. Adding such a stabilizer to titanium produces an alloyhaving a lower transus temperature between the lower-temperature alphaphase and the higher-temperature beta phase than the transus for puretitanium (about 882° C.). Consequently, adding a sufficient amount ofbeta stabilizer to alpha-phase titanium causes at least some of thealpha-phase material to transform to the beta phase, with an attendantchange in specific volume of the transformed material. Removing thebeta-stabilizer from the material reverses the transformation. In apreferred embodiment, superplasticity is induced in a titanium-basedworkpiece material by changing the concentration of hydrogen therein.

The invention is not limited to transformations accessible by thethermal pathways of the prior art but also enables superplastic behaviorto be induced by other transformations, not accessible throughtemperature change alone. The method of the invention is generallyapplicable to materials susceptible upon change in chemical compositionto a phase transformation that generates the strain increment. Some suchcycles may be executable isothermally. However, the method of theinvention also encompasses process pathways that include temperaturechange in addition to the chemical cycling, whether the temperaturechange occurs simultaneously with or sequentially to the chemicalchange. The thermal variation may be actively imposed on the workpieceor originate within the workpiece due to the imposed change in chemicalconcentration.

In a preferred embodiment, hydrogen concentration is changed in atitanium-based material to cause alternate precipitation and dissolutionof a second, titanium hydride phase. Titanium hydride precipitates whenhydrogen is added to titanium in excess of the hydrogen solid solubilitylimit. The relatively high specific volume of the hydride phasetranslates into a molar volume mismatch on the order of 17% with respectto the original titanium. The volume mismatch generates sufficientinternal stress to produce very large superplastic deformation. In oneembodiment, the original workpiece material comprises a single phase. Inan alternative embodiment, the workpiece material is a multiphasecomposite including a matrix of one or more phases and one or moreadditional phases. In a preferred embodiment, the change in chemicalcomposition of the workpiece material alternately induces and reverses aphase transition in one or more transformable phases, which may be anadditional phase or part of the matrix. The composite may also includeone or more phases not subject to phase transformation upon the changein chemical composition. The phase distribution is selected so as toallow forward and reverse phase transformation without interfacialdecohesion. Bonding between the composite's phases may contribute to theinternal stress caused by the phase transition.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing discussion will be understood more readily from thefollowing detailed description of the invention, when taken inconjunction with the accompanying drawings:

FIG. 1 is a portion of the phase diagram for the titanium-hydrogensystem; and

FIG. 2 graphically depicts the accumulation of superplastic strain in atitanium workpiece under tensile stress during hydrogen cycling.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The use of hydrogen concentration cycling to induce superplasticity in atitanium-based material is demonstrated with reference to FIG. 1. Inpure, hydrogen-free titanium the alpha and beta phases exist overmutually distinct temperature ranges separated by the transustemperature of 882° C. At nonzero concentrations of hydrogen intitanium, the stability field 10 of the higher-temperature beta phaseextends to temperatures lower than 882° C. and overlaps the temperaturerange of the stability field 20 of the lower-temperature alpha phase.Increasing the hydrogen concentration of a volume of alpha-phasematerial initially in a state 24--either along direction A orisothermally along direction B--into the two-phase field 30 causesconversion of a segment of the alpha-phase volume to the beta phase. Theextent of the segment increases with the overall hydrogen content of thevolume until its composition lies in the beta phase field 10.

A nominally pure titanium sample was superplastically deformed bychemically induced alpha-beta transformation. The sample was brought to808° C. by radiative heating. The sample was held in an argonenvironment and, within the alpha stability field 20 well below thetransus temperature, subjected to a uniaxial tensile stress of 2.5 MPa.Hydrogen was then provided to the heated sample in tension by adding 4%hydrogen gas in the argon stream. This gas-phase hydrogen concentrationwas maintained for 600 seconds, after which hydrogen was withdrawn fromthe sample by restoring the pure argon stream for 600 seconds. FIG. 2shows the strain increment present after this 1200 second cycle due tothe difference in specific volume between the alpha and beta phases. Atotal of nine hydrogen concentration cycles were applied to the samplein tension which was maintained at a constant temperature throughout thecycling. As illustrated in FIG. 2, additional strain accumulates witheach cycle. The total strain was over 12%, corresponding to about 1.4%per cycle, which is much greater than deformations seen in identicalsamples maintained under the same conditions in an argon orargon-hydrogen atmosphere without chemical cycling.

Many variations of this process are within the scope of the invention.For titanium, a tensile stress up to about 10 MPa or even higher may beused, the strain introduced per cycle increasing with applied stress.Additional steps may be included. For example, when the desireddeformation has been achieved, residual hydrogen may be removed byvacuum annealing if desired.

Chemically induced superplasticity using hydrogen is also appropriatefor workpiece materials other than pure titanium. For example, hydrogensimilarly affects phase relationships in titanium-based materials, forexample titanium alloys such as Ti6Al4V. Other allotropic metals such aszirconium, neodymium, lanthanum, strontium, and uranium and their alloysalso show phase relationships that allow chemical induction ofsuperplasticity by cycling hydrogen concentration. Allotropic andnonallotropic metals that form hydrides with mismatch with respect tothe host metal matrix--such as titanium, zirconium, niobium, tantalumand vanadium--are deformable through chemically induced superplasticityby addition of hydrogen under hydride-forming conditions. In the case oftitanium, such a process converts a segment of the workpiece to thedelta phase, which, with reference again to FIG. 1, has single-phasestability field 40. (This approach is easily combined withmicrostructure refinement of titanium, by cyclic hydriding anddehydriding, for improving its room-temperature properties.)

Chemical composition may also be changed reversibly using nitrogen oroxygen in materials based on, respectively, nitride or oxide ceramics,or based on allotropic metals such as iron, titanium, zirconium, andyttrium. Carbon may be delivered to an iron-based workpiece material bya gas such as methane and then removed by reaction with a gas such ashydrogen or oxygen.

It will therefore be seen that the foregoing represents a highlyadvantageous approach to inducing superplastic deformation. The termsand expressions employed herein are used as terms of description and notof limitation, and there is no intention, in the use of such terms andexpressions, of excluding any equivalents of the features shown anddescribed or portions thereof, but it is recognized that variousmodifications are possible within the scope of the invention claimed.

What is claimed is:
 1. A method of inducing superplasticity in aworkpiece, the workpiece being of a material susceptible to a phasetransformation, upon change in concentration therein of a chemicalcomponent and at a temperature, the method comprising the steps of:a.bringing the workpiece to the temperature; and b. alternately providingthe chemical component to and removing the chemical component from theworkpiece while the workpiece is subject to a biasing stress, therebyalternately inducing and reversing the phase transition to introduce astrain increment and produce a change in an overall dimension of theworkpiece, due to the strain increment, of at least 0.5%.
 2. The methodof claim 1 wherein a cycle of inducing and reversing the phasetransition produces a change of at least one-half percent in an overalldimension of the workpiece.
 3. The method of claim 2 wherein the biasingstress is tensile.
 4. The method of claim 1 further comprising repeatingstep b. at least once, each repetition introducing a strain increment,the change in the overall dimension of the workpiece being due toaccumulation of strain increments, the change in an overall dimension ofthe workpiece corresponding to an average strain increment of at leastone-half percent per repetition.
 5. The method of claim 4 wherein thebiasing stress is tensile.
 6. The method of claim 4 wherein the phasetransition comprises formation of a compound containing an element ofthe chemical component and an element of the material, the alternateprovision and removal of the chemical component alternately forming anddissolving the compound, the change in an overall dimension of theworkpiece corresponding to an average strain increment of at least 1.5%per repetition.
 7. The method of claim 1 wherein the change in theoverall dimension is at least 1.0%.
 8. The method of claim 1 wherein thechange in the overall dimension is at least 1.5%.
 9. The method of claim1 further comprising repeating at least once the alternate provision ofthe chemical component to and removal of the chemical component from theworkpiece while the workpiece is subject to a biasing stress, eachrepetition introducing a strain increment, the change in the overalldimension of the workpiece being due to accumulation of strainincrements.
 10. The method of claim 9 wherein the change in the overalldimension of the workpiece is at least 1.5%.
 11. The method of claim 9wherein the change in the overall dimension of the workpiece is at least12%.
 12. The method of claim 1 further comprising repeating at leastonce the alternate provision of the chemical component to and removal ofthe chemical component from the workpiece while the workpiece is subjectto a biasing stress, each repetition introducing a strain increment, thechange in the overall dimension of the workpiece being due toaccumulation of strain increments and equal to at least 1.5% perrepetition.
 13. The method of claim 1 further comprising repeating atleast once the alternate provision of the chemical component to andremoval of the chemical component from the workpiece while the workpieceis subject to a biasing stress, each repetition introducing a strainincrement, the change in the overall dimension of the workpiece beingdue to accumulation of strain increments and equal to at least 0.5% perrepetition.
 14. The method of claim 1 wherein the biasing stress isnoncompressive.
 15. The method of claim 1 wherein the biasing stress istensile.
 16. A method of inducing superplasticity in a workpiece, theworkpiece being of a material susceptible to a phase transformation,upon change in concentration therein of a chemical component and at atemperature, the phase transformation comprising formation of a compoundcontaining an element of the chemical component and an element of thematerial, the method comprising the steps of:a. bringing the workpieceto the temperature; and b. alternately providing the chemical componentto and removing the chemical component from the workpiece while theworkpiece is subject to a tensile biasing stress, thereby alternatelyforming and dissolving the compound to introduce a strain increment andproduce a change in an overall dimension of the workpiece due to thestrain increment.
 17. The method of claim 16 wherein the compound is ahydride.
 18. The method of claim 17 wherein the compound is a titaniumhydride.
 19. The method of claim 17 wherein the material includes aphase of niobium, tantalum or vanadium or of an alloy based thereon. 20.The method of claim 17 wherein the material includes a phase ofzirconium or of an alloy based thereon.
 21. The method of claim 16further comprising repeating step b. at least once, each repetitionintroducing a strain increment, the change in the overall dimension ofthe workpiece being due to accumulation of strain increments.
 22. Amethod of inducing superplasticity in a workpiece, the workpiece beingof a material susceptible to a phase transformation, upon change inconcentration therein of a chemical component and at a temperature, themethod comprising the steps of:a. bringing the workpiece to thetemperature; and b. alternately providing the chemical component to andremoving the chemical component from the workpiece while the workpieceis subject to a tensile biasing stress, thereby alternately inducing andreversing the phase transition to introduce a strain increment andproduce a change in an overall dimension of the workpiece due to thestrain increment.
 23. The method of claim 22 further comprisingrepeating step b. at least once, each repetition introducing a strainincrement, the change in the overall dimension of the workpiece beingdue to accumulation of strain increments.
 24. The method of claim 23wherein the workpiece is of a titanium-based material, the componentbeing hydrogen.
 25. The method of claim 22 further comprising the stepof shaping the workpiece to produce a change in shape of the workpieceby accumulation of superplastic strain increments.
 26. The method ofclaim 22 wherein the workpiece is of a composite material comprising amatrix and one or more additional phases, the composite material havinga transformable phase susceptible to a phase transition upon change inconcentration therein of a chemical component at a temperature, thealternate provision and removal of the chemical component alternatelyinducing and reversing the phase transition in the transformable phase.27. A method of inducing superplasticity in a workpiece, the workpiecebeing of a material susceptible to a phase transformation, upon changein concentration therein of a chemical component and at a temperature,the method comprising the steps of:a. bringing the workpiece to thetemperature; b. applying an external tensile stress of at least 2.5 MPato the workpiece to subject the workpiece to a biasing stress; and c.alternately providing the chemical component to and removing thechemical component from the workpiece while the workpiece is subject tothe tensile biasing stress, thereby alternately inducing and reversingthe phase transition to introduce a strain increment and produce achange in an overall dimension of the workpiece due to the strainincrement.
 28. The method of claim 27 wherein the external tensilestress is at least 10 Mpa.
 29. A method of inducing superplasticity in aworkpiece, the workpiece being of a material susceptible to a phasetransition, upon change in concentration therein of a chemical componentand at a temperature, the method comprising the steps of:a. bringing theworkpiece to the temperature; and b. alternately providing the chemicalcomponent to and removing the chemical component from the workpiecewhile the workpiece is subject to a noncompressive biasing stress,thereby alternately inducing and reversing the phase transition, in amanner that introduces a strain increment and produces a change in anoverall dimension of the workpiece due to the strain increment.
 30. Themethod of claim 29 further comprising the steps of:a. repeating at leastonce the step of alternately providing the chemical component to andremoving the chemical component from the workpiece comprises, therebyrepeatedly inducing and reversing the phase transition, so that eachrepetition introduces a superplastic strain increment; and b. shapingthe workpiece to produce a change in shape of the workpiece byaccumulation of superplastic strain increments.
 31. The method of claim29 wherein the workpiece is of a composite material comprising a matrixand one or more additional phases, the composite material having atransformable phase susceptible to the phase transition upon change inconcentration therein of the chemical component at the temperature. 32.The method of claim 29 wherein the workpiece is of a titanium-basedmaterial susceptible to the phase transition at a temperature, thechemical component being hydrogen.
 33. The method of claim 29 whereinthe phase transition comprises formation of a compound containing anelement of the chemical component and an element of the material, thealternate provision and removal of the chemical component alternatelyforming and dissolving the compound.
 34. A method of inducingsuperplasticity in a workpiece comprising a material, the methodcomprising altering the concentration in the workpiece of a chemicalcomponent while the workpiece is subject to a noncompressive biasingstress, thereby introducing a strain increment into the workpiece andproducing a change in an overall dimension of the workpiece due to thestrain increment.
 35. A method of inducing superplasticity in aworkpiece comprising a material, the method comprising altering theconcentration in the workpiece of a chemical component while theworkpiece is subject to a biasing stress, thereby introducing a strainincrement into the workpiece and producing a change in an overalldimension of the workpiece due to the strain increment, said change inan overall dimension of the workpiece comprises expanding internalcavities in the workpiece, thereby foaming the material.
 36. The methodof claim 35 wherein the workpiece is of a material susceptible to aphase transition, upon change in concentration therein of the chemicalcomponent and at a temperature, further comprising the step of bringingthe workpiece to the temperature, altering the concentration in theworkpiece of a chemical component comprising alternately providing thechemical component to and removing the chemical component from theworkpiece.
 37. A method of inducing superplasticity in a workpiececomprising a material, the method comprising altering the concentrationin the workpiece of carbon while the workpiece is subject to a biasingstress, thereby introducing a strain increment into the workpiece andproducing a change in an overall dimension of the workpiece due to thestrain increment.
 38. The method of claim 37 wherein the workpiece is ofa material susceptible to a phase transition, upon change inconcentration therein of carbon and at a temperature, further comprisingthe step of bringing the workpiece to the temperature, altering theconcentration in the workpiece of carbon comprising alternatelyproviding carbon to and removing carbon from the workpiece.
 39. Themethod of claim 38 wherein the material includes a phase of iron or ofan alloy based thereon.
 40. A method of inducing superplasticity in aworkpiece comprising a material, the method comprising altering theconcentration in the workpiece of a chemical component while theworkpiece is subject to a biasing stress, thereby introducing a strainincrement into the workpiece and producing a change in an overalldimension of the workpiece due to the strain increment, the chemicalcomponent being oxygen or nitrogen.
 41. The method of claim 40 whereinthe workpiece is of a material susceptible to a phase transition, uponchange in concentration therein of the chemical component and at atemperature, further comprising the step of bringing the workpiece tothe temperature, altering the concentration in the workpiece of thechemical component comprising alternately providing the chemicalcomponent to and removing the chemical component from the workpiece. 42.The method of claim 41 wherein the material includes a phase of iron,zirconium, titanium, or yttrium or of an alloy based thereon.
 43. Themethod of claim 41 wherein the material includes a phase of an oxideceramic or a nitride ceramic.